Source : Webster's Revised Unabridged Dictionary (1913)

Preposition \Prep`o*si"tion\, n. [L. praepositio, fr. praeponere
   to place before; prae before + ponere to put, place: cf. F.
   pr['e]position. See {Position}, and cf. {Provost}.]
   1. (Gram.) A word employed to connect a noun or a pronoun, in
      an adjectival or adverbial sense, with some other word; a
      particle used with a noun or pronoun (in English always in
      the objective case) to make a phrase limiting some other
      word; -- so called because usually placed before the word
      with which it is phrased; as, a bridge of iron; he comes
      from town; it is good for food; he escaped by running.

   2. A proposition; an exposition; a discourse. [Obs.]

            He made a long preposition and oration. --Fabyan.

Source : WordNet®

preposition
     n 1: a function word that combines with a noun or pronoun or noun
          phrase to form a prepositional phrase that can have an
          adverbial or adjectival relation to some other word
     2: (linguistics) the placing of one linguistic element before
        another (as placing a modifier before the word it modifies
        in a sentence or placing an affix before the base to which
        it is attached)